The present invention relates to dynamic ground compaction techniques. These techniques are used to improve the structural characteristics of the ground, in particular prior to building construction works.
A dynamic compaction treatment densifies the ground down to great depths by means of very high energy waves. It involves heavy loads, typically from 10 to 100 tons, falling from a height of typically 10 to 40 meters. The layout of the impact points on the ground and the other parameters of the treatment (energies, phasing, rest periods) depend on the characteristics of the soil to be treated and possibly on measurement results obtained in a trial zone. These parameters are determined beforehand based on the desired ground characteristics.
Such ground treatment is frequently used for the foundation of buildings, or to stabilized large areas of embankment work or loose soil.
Two general types of dynamic ground compaction methods can be distinguished:
1) Method with Follower Cables.
Cable shovels used for dragline works are frequently equipped with winches having clutch means providing a so-called “free fall” function. Such machine can be used for dynamic ground compaction, by attaching the compaction load to one or more winch cables. After actuation of the winches to hoist the load up to the desired height, the clutches are released and the load falls, driving the cable and the winch drum behind it. After the impact, the winches are braked to stop their rotation, the cables are pulled again and a new cycle is resumed.
A shortcoming of that method is that, with the civil engineering machines available on the market, it is observed that the energy imparted to the ground on the impact is only 50 to 60% of the potential energy accumulated when hoisting the load. This low efficiency is due to frictional losses and to the inertia of the cables and winches. Such method can only be applied by using a single cable per winch (no multiplication of the winch effort) and a single cable layer on the winch drum. In practice, this limits the falling height to about 25 m and the compaction loads to about 25 tons. Accordingly, the unitary impact energy is at most 60%×25,000×9.81×25≈3,700 kJ.
2) Free Falling Method
To alleviate the poor falling efficiency of the above method, a possibility is to use a hoisting machine equipped with a connection device which can be released when loaded and which is interposed between the compaction load and the cables. Such connection device may be of the hook type, as used for towage. It can also be a specially-designed hydraulic clamp. The compaction load is hoisted up to the desired height where the winches are stopped, and then the hook or clamp releases the load which really falls freely.
The main advantage of that method is its high efficiency since the impact energy is equal to the potential energy produced by the hoisting action. In addition, it is possible to use reeving systems to multiply the traction force applied by the winches. It is also possible to use more than one cable layers on the winch drum. The impact energy is basically limited by the stability of the hoisting machine when loaded.
However, the method also has a number of drawbacks. When the connection means are released, the elastic energy built up within the machine and the cables when hoisting the load is suddenly transmitted to the connection device, mainly by the reaction of the cables. The mobile parts consisting of the connection device and possibly of the reeving system are kicked upwardly with a considerable energy. They can also be shoved laterally due to the dissymmetry of the system. Such reaction can cause various troubles, such as derailment of the cables, impacts on the crane structure, etc. The phenomenon has to be compensated for, either by increasing the weight of the moving parts up to about 20% of the weight of the release load, to the detriment of the overall efficiency, or by using external moors to limit the movements of the connection device.
In addition, the lowering of the connection device for reconnection to the load on the ground takes a significant amount of time, since it depends on the speed capacity of the unloaded winches, which is usually low. At best, a lowering time of the same order as the hoisting time can be expected. Therefore, this second method is relatively time-consuming.
An object of the present invention is to alleviate the above-commented drawbacks of the prior art.